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How to buy the best external hard drive

By Jon Barrow

Want to back up your precious digital photos and documents? An external hard drive could be the solution. Read on to find out how to choose one.

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What is an external hard drive and what does it do?

An external hard drive is a portable device that you can easily plug in to your computer to add extra storage or to transfer files.

A hard drive can be a computing lifesaver for plenty of reasons. If there isn't much storage on your computer itself, then connecting an external hard drive can give you the storage space you need to work with your files or store videos and photos.

Most importantly, an external drive enables you to make backup copies of important files that you can store away from your computer. Should your laptop or home computer fail - or if it's stolen, or infected by ransomware - then you'll still be able to access priceless photos and home videos.

As most external hard drives are portable, they offer an easy way to move your files from one computer to another.

What size hard drive do I need?

External hard drives typically come with between 250GB (gigabytes) and 4TB (terabytes - effectively 4000GB) of data. Even at the smaller end of the scale, external hard drives can hold thousands of your files.

To put it in context, on average, 1GB can hold about 500 iPhone photos or about 200 iTunes digital songs.

As such, you'll probably only need to spend more on a very large capacity device if you want to back up your entire computer, or if you store a lot of video and audio files.

Solid state drives versus regular hard drives

There are two types of external hard drives - solid state drives (SSDs) or traditional hard disk drives (HDDs).

SSDs use flash memory - the same type of storage used in SD cards for digital cameras or in iPads and other tablets. The lack of moving parts means they're less likely to break down than more mechanical HDDs and ensures that they're ready to go straight away and can retrieve files much more quickly.

They're also lighter, quieter and use less battery power, so won't drain your laptop battery as quickly when connected.

HDDs use older technology, with a spinning disc inside that physically moves. It takes longer to copy your files to this type of drive, but they offer two big advantages over SSDs – they're cheaper and offer far larger storage capacities. SSDs typically have around 256GB of storage capacity, while HDDs may have as much as 2TB or even 4TB.

We'd advise you to go with a traditional HDD if price is important to you, and if you have a large number of files to store. But, if you're not storing thousands of videos or backing up your entire system, an SSD could prove more reliable and a faster storage tool - at a higher cost than an HDD.

Hard drive connections and data speeds explained

External hard drives are usually connected to your computing device using a USB cable. These are easy to use – you don't have to set anything up or install special software; you just plug it in and the drivers will install automatically to let the two devices communicate. The computer recognises the drive, and you're able to read and save files almost instantly.

External hard drives typically come with a USB cable with a small connector on one end - either a micro USB or a newer Type C - that you plug in to the drive, and a larger, rectangular Type A connector on the other (to plug in to your computer).

The transfer speeds on offer are determined by the USB standard on both your computer and on the external drive. Choosing a device that uses the latest standards – USB C – will ensure you get the best performance.

The most common standards are USB 2, which has a maximum speed of 480Mbps, and USB 3, which can go as fast as 5Gbps (more than 10 times as much). Both of these have a 'standard fit' USB connector. The newer USB C standard is even faster - it doubles the maximum theoretical data transfer speed to 10Gbps. However, it uses a completely different connection shape, so your computer will need a USB C port.

In practical terms, though, you're unlikely to achieve the speeds stated above. Numerous factors can slow down the data transfer speed you can actually get, including the size and type of file, the processor on your computer, and how many other tasks you're conducting at the same time.

USB 3 (and 3.1) is backwards-compatible, so a USB 3.0 hard drive and cable will work with a computer that only has USB 2.0 ports - but it won't give you the full potential speeds the hard drive is capable of.

Portable vs desktop drives

Some hard drives are designed to be kept near your computer - these are larger and require their own power supply. The benefit is that they normally have very large storage capacities. This type of hard drive is becoming rarer, however, as these days it's possible to fit huge storage capacities into very portable hard drives.

Portable drives – such as the ones we've tested – are smaller and typically will fit in your bag or even your pocket. Because they're powered through the USB connection to the computer they don't require their own power source.

Back-up software and other extra features

Some external hard drives have built-in software that will automatically back up your digital files. This isn't essential – it's easy to make backups manually, even by dragging and dropping files over. But it can be handy to automate this process, as it means you'll never forget to save your precious documents. Though backing things up can slow down your computer, you can set it to happen when you're unlikely to be using it.

Drives can also come with pre-installed security software that will password protect your data. This can be valuable, as external drives can end up storing sensitive documents or personal photos that you wouldn't want to fall in to the wrong hands.